HF surgical appliances are known for a plurality of possible applications. With some of these possible applications, the load impedance lies between 0.1 and 1 kΩ, with other possible applications, the load impedance can be even lower. In particular, when a transurethral resection of the prostate (TUR) is to be performed with a bipolar instrument with a highly conductive irrigant fluid (NaCl 0.9%), the load impedance is only about 20Ω.
To be able to perform a load adaptation with a conventional HF surgical appliance, as shown in FIG. 4, a generator G is provided in the HF surgical appliance (indicated in FIG. 4 with a broken line); the generator G comprises an output transformer T1 that supplies, via an inductance L1 and a capacitance C1, which together form a resonant circuit, output terminals 1, 2, on which the load resistor ZL is suspended. With a high-resistance load, a first switch S1 is closed so that the entire secondary winding of the transformer T1 is coupled to the load resistor ZL via the resonant circuit. In the case of a low load impedance ZL, only a part of the secondary winding of the transformer T1 is coupled via the switch S2 to the load ZL. In this case, although an improvement of the adaptation conditions of the impedances is ensured, the transformer T1 has to be provided with a winding that has a larger wire cross section than the rest of the windings, or has an even larger overall wire cross section, to tap the switch S2. This is obviously a drawback. Furthermore, the adaptation of the resonant circuit, which has to take place in dependence on the generator frequency and the load ZL, no longer takes place in one or the other switch position.